Eeviews — Pictefa Jurassic and Cretaceotis Rocks. 231 



The line of separation between the two periods has been placed 

 between stages 2 and 3, but M. Pictot regards it as a very feeble 

 limit. 



The four stages are bound together by great palneontological 

 analogies. Several species pass undoubtedly from No. 1 to No. 2, 

 from that to No. 3, and from No. 3 to No. 4, Nos. 2 and 3 in par- 

 ticular have about one-third of their species in common ! lie, how- 

 ever, suspends judgment until new sections are known and new local 

 faunas worked out, which he hopes may determine the question. A 

 minute description of the fauna, with Terehratula diphya, of the 

 Tyrol, will soon be ready, and ho expects that the results of obser- 

 vations by M. M. de Verneuil and Ernest Favre in Andalusia, will 

 supply some desiderata towards a definite solution. 



M. Pictet concludes with some theoretical reflections on the man- 

 ner in which these researches must be received and discussed. 

 He believes that the development and succession of beings has 

 been, and still is, under the control of laws perfectly regular, and 

 that the faunas have been modified more or less gradually, and with 

 a certain slowness, under the influence of varied circumstances, like 

 those which bring about analogous effects in modern seas. 



But as these circumstances are not identical at all times, nor in all 

 places, it is not at all probable that changes happen exactly in the 

 same manner in all geological basins. 



The history of the succession of life must consequently be very 

 complex, and modified by local causes ; it could not be drawn up 

 very simply, nor would it be applicable to all cases. 



M. Pictet makes a few remarks on the use of the word normal, 

 objecting to its application to particular beds in the " Anglo-French 

 basin," in which according to some Naturalists are to be found the 

 normal succession of stages; while the beds in other localities, which 

 are not rigorously identical in character, are considered abnormal. 



If by chance, as M. Merian has remarked, the first development of 

 geology had taken place elsewhere, the classification would doubtless 

 have been different. Breaks that in the " Anglo-French basin " are 

 very clear, would in other places be represented by a continued suc- 

 cession of beds, and vice versa. 



M. Pictet remarks on the indefinite character of Zones. They are 

 useful, but they should not be characterized by one or two isolated 

 fossils, but rather by an assemblage of forms. 



Eecent discoveries ever tend to soften down the hard lines that 

 were supposed to separate our great formations. The limits as- 

 signed to them are mere inventions of science, useful in their way 

 as points of reference, sometimes indicating local breaks, but never 

 an index of change for localities. 



The passages of the Devonian into the Carboniferous are now well 

 established, the Ehaetic beds fill up the gap between the Trias and 

 Lias, new freshwater deposits discovered by M. Matheron connect 

 the Cretaceous and Tertiary periods, and the latter merges insensibly 

 into the Quaternary and Modern periods. 



